Allow pg_basebackup to stream transaction log in tar mode

This will write the received transaction log into a file called
pg_wal.tar(.gz) next to the other tarfiles instead of writing it to
base.tar. When using fetch mode, the transaction log is still written to
base.tar like before, and when used against a pre-10 server, the file
is named pg_xlog.tar.

To do this, implement a new concept of a "walmethod", which is
responsible for writing the WAL. Two implementations exist, one that
writes to a plain directory (which is also used by pg_receivexlog) and
one that writes to a tar file with optional compression.

Reviewed by Michael Paquier

src/bin/pg_basebackup/walmethods.h

@@ -0,0 +1,45 @@
+/*-------------------------------------------------------------------------
+ *
+ * walmethods.h
+ *
+ * Portions Copyright (c) 1996-2016, PostgreSQL Global Development Group
+ *
+ * IDENTIFICATION
+ *		  src/bin/pg_basebackup/walmethods.h
+ *-------------------------------------------------------------------------
+ */
+
+
+typedef void *Walfile;
+
+typedef enum
+{
+	CLOSE_NORMAL,
+	CLOSE_UNLINK,
+	CLOSE_NO_RENAME,
+}	WalCloseMethod;
+
+typedef struct WalWriteMethod WalWriteMethod;
+struct WalWriteMethod
+{
+	Walfile(*open_for_write) (const char *pathname, const char *temp_suffix, size_t pad_to_size);
+	int			(*close) (Walfile f, WalCloseMethod method);
+	bool		(*existsfile) (const char *pathname);
+	ssize_t		(*get_file_size) (const char *pathname);
+
+	ssize_t		(*write) (Walfile f, const void *buf, size_t count);
+	off_t		(*get_current_pos) (Walfile f);
+	int			(*fsync) (Walfile f);
+	bool		(*finish) (void);
+	char	   *(*getlasterror) (void);
+};
+
+/*
+ * Available WAL methods:
+ *	- WalDirectoryMethod - write WAL to regular files in a standard pg_xlog
+ *	- TarDirectoryMethod - write WAL to a tarfile corresponding to pg_xlog
+ *						   (only implements the methods required for pg_basebackup,
+ *						   not all those required for pg_receivexlog)
+ */
+WalWriteMethod *CreateWalDirectoryMethod(const char *basedir, bool sync);
+WalWriteMethod *CreateWalTarMethod(const char *tarbase, int compression, bool sync);